In the oil and gas industry, water well and injector well industry, boreholes are drilled into subterranean formation that are capable of the flow of fluids through the formation. The flow may be through the permeability of the formation, through vugs or pore spaces, through natural or manmade fractures. Additional flow paths may also exist such as an open hole wellbore, casing, perforation tunnels, slots, tubing, annuli, washouts, particulate packs, screens and completion/service/workover equipment.
While during the producing (or injecting) phase of a well it is generally desired to be as free flowing as possible or at a rate set by the design of the well. However, during the drilling, completion or workover phases of the well it may be desired to allow flow of fluids into only a portion of the formation or wellbore while preventing it from flowing into other portions. In many cases, particulate material has been employed to bridge or seal those flow paths wherein the flow of fluids is undesirable. Although a large number of materials have been used historically for this purpose, they can generally be classified as permanent, removable or temporary.
The action of the particulate material may be described in a number of ways. Terms such as bridging, sealing, plugging, filtercake forming, diverting, fluid loss, lost circulation, permeability modification, packing and coating have commonly been used in the literature.
Examples of permanent materials used are sand, clays, barite and a number of waste stream materials such as fly ash. Although these materials function to control fluid loss at the time of use, they may have severe residual effect on the production or injection of the well later on.
An example of a removable material is calcium carbonate. At some point, a cleanup solution such as acid in placed in contact with the calcium carbonate to remove it from flow paths.
Examples of temporary materials are those that degrade, decompose or have gradual solubility in the wellbore fluid. Biodegradable polymers such as polylactic acid are commonly used since they degrade in the presents of even a small amount of water. However, these polymers may have temperature and cost limitation for many wellbore applications.